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This study demonstrates the feasibility of speleothem magnetism as a paleo-hydrology proxy in speleothems growing in semi-arid conditions. Soil-derived magnetic particles in speleothems retain valuable information on the physicochemical conditions of the overlying soil, and changes in bedrock hydrology. Yet, the link between magnetic and isotopic proxies of speleothems has been only partly established. We reveal strong coupling between the inflow of magnetic particles (quantified using the magnetic flux index, IRMflux) and δ13C in two Holocene speleothems from Soreq Cave (Israel). The stalagmite record spans from ca. 9.7 to ca. 5.4 ka, capturing the warm-humid conditions associated with the early Holocene and the transition to mid-Holocene wet-dry cycles. Extremely low IRMflux during the early Holocene, indicating minimal contribution from the overlying soil, is accompanied by anomalously high δ13C (approaching bedrock values) hypothesized to be caused by high rainfall and soil erosion. By contrast, IRMflux during the mid-Holocene covaries with the saw-tooth cyclicity of δ13C and δ18O, interpreted as rapid fluctuations in rainfall amount. The peaks in IRMflux precede the negative (wet) δ13C peaks by ~60–120 yr. The apparent lag is explained as a rapid physical translocation of overlying soil particles via groundwater (high IRMflux) as a response to increasing rainfall, compared with slower soil organic matter turnover rates (10–102 yr).

The accumulation of heavy minerals along a part of the southeastern Brazilian coast (northern coast of Rio de Janeiro State, Brazil) provides insight into sedimentary provenance and transport by longshore drift. In particular, recent work has focused on heavy minerals as tracers to determine sedimentary sources and transport pathways, indicating a mineral distribution according to the northward and southward coastal transport cells. There is also evidence of heavy-mineral transport by the Paraíba do Sul River. However, research has not been done in this area on the opaque fraction (iron oxides and oxyhydroxides), which are good tracers for provenance and distribution processes. Here we intend to: 1) characterize the spatial variation of the magnetic properties of heavy-mineral contents throughout a coastal area of the Itabapoana and Paraíba do Sul River mouths (state of Rio de Janeiro), 2) evaluate the contribution of coastal dynamics on the mineral distribution along this coastal area, 3) provide contributions on the provenance of iron-bearing minerals. Results indicate that a bimodal pattern was observed for the magnetic remanence of magnetite and hematite near the Paraíba do Sul River. For the northernmost section, goethite exhibits a gradual increase in concentration towards the central part of the study area. Additionally, higher heavy-mineral content is observed at the northern coastal section, which may be related to coastal morphology, which is exposed to high-energy beach processes. It is possible to verify three sampling groups in accordance with their magnetic properties: 1) the “southernmost” and 2) the “northernmost” groups, suggesting a mineral provenance related to the Paraíba do Sul and Itabapoana rivers, respectively, as possible major sources, and 3) another at the intermediate section of the area, possibly linked to the erosion of the Barreiras Formation bluffs. Results also show a correspondence between the variability of magnetic parameters and the main directions of the coastal transport cells.

The Duluth Complex, Northeastern Minnesota, contains sulfide-rich magmatic intrusions that, collectively, represent one of the world’s largest known economic deposits of copper, nickel, and platinum group elements (Cu-Ni-PGEs). Previous work showed that microbial communities associated with experimentally-weathered Duluth Complex waste rock and tailings were dominated by uncultivated taxa and other populations not typically associated with mine waste. However, those experiments were designed for kinetic testing and do not necessarily represent the conditions expected for reclaimed mine waste or long-term weathering in the environment. We therefore used 16S rRNA gene methods to characterize the microbial communities present on the surfaces of naturally-weathered and historically disturbed outcrops of Duluth Complex material, as well as a circumneutral seep draining a reclaimed blast pit. Rock surfaces were dominated by diverse uncultured Ktedonobacteria, Acetobacteria, and Actinobacteria while seeps were dominated by Proteobacteria, including Leptothrix spp. and Methylovulum spp. All samples had abundant algae and other phototrophs. These communities were distinct from previously-described microbial assemblages from experimentally-weathered Duluth Complex rocks, suggested different energy and nutrient resources in the reclaimed rocks, outcrops, and seeps. Sulfide mineral incubations performed with and without algae showed that photosynthetic microorganisms could have an inhibitory effect on some of the autotrophic populations from the site, resulting in slightly lower sulfate release and differences in the dominant microorganisms. The microbial assemblages from these weathered outcrops show how communities are expected to develop during natural weathering of sulfide-rich Duluth Complex rocks, and represent baseline data that could be used to evaluate the effectiveness of future reclamation of tailings and waste rock produced by large scale mining operations.

Caves occur everywhere on our planet, from the tropics to the high latitudes and from below sea level to alpine settings. Cave morphologies provide clues to their formation mechanisms, and their iconic mineralogical features—stalagmites and stalactites—carry a wealth of paleoenvironmental information encoded in their geochemistry and mineralogy. Recent work demonstrates a striking improvement in our ability to decode these paleoenvironmental proxies, and dramatic geochronological advances enable higher resolution records that extend further back in geologic time. Cave research addresses an ever-increasing range of geoscience problems, from establishing the timing and mechanisms of climate change to uncovering detailed records of geomagnetic field behavior.

No matter how quiet and pristine a cave setting may appear, all speleothems contain assemblages of magnetic minerals. These iron oxide minerals are derived largely from overlying soils, though minor fractions may come from the residuum of dissolved bedrock, reworked sediment carried by episodic floods, geomicrobiological activity, and even windblown dust. Regardless of their origin, these minerals become aligned with Earth’s ambient magnetic field before they are fixed within a speleothem’s growing carbonate matrix. Here, we describe how the magnetism of stalagmites and flowstone can be used to chronicle high-resolution geomagnetic behavior and environmental change.

We provide a comprehensive overview of historic chloride concentrations in the groundwater of the Twin Cities metropolitan area (TCMA) in Minnesota, in order to define the extent of chloride contamination, due primarily to the seasonal application of deicing salt to roadways. Data collected from 1278 wells between 1965 and 2020 are representative of the major aquifers underlying the TCMA and establish a regional natural background chloride concentration of less than 10 mg/L. However, 55% of all measurements (1616 of 2943) are above 10 mg/L, with the highest concentrations found within the uppermost Quaternary aquifers. Chloride concentrations in underlying bedrock aquifers are negatively correlated with the thickness and clay composition of overlying materials. Most chloride measurements (92%) remain below chronic exposure limits set by state and federal authorities. Historical trends indicate that, if the current imbalance between chloride inputs and outflows persists, chloride concentrations in TCMA aquifers will surpass regulatory thresholds by midcentury as surface waters and Quaternary aquifer waters migrate into underlying bedrock aquifers. Most wells in this study are monitored annually, making it impossible to detect important sub-annual fluctuations of chloride concentration that can exceed 40%.

Speleothems can provide high-quality continuous records of the direction and relative paleointensity of the geomagnetic field, combining high precision dating (with U-Th method) and rapid lock-in of their detrital magnetic particles during calcite precipitation. Paleomagnetic results for a mid-to-late Holocene stalagmite from Dona Benedita Cave in central Brazil encompass ~1900 years (3410 BP to 5310 BP, constrained by 12 U-Th ages) of paleomagnetic record from 58 samples (resolution of ~33 years). This dataset reveals angular variations of less than 0.06° yr−1 and a relatively steady paleointensity record (after calibration with geomagnetic field model) contrasting with the fast variations observed in younger speleothems from the same region under influence of the South Atlantic Anomaly. These results point to a quiescent period of the geomagnetic field during the mid-to-late Holocene in the area now comprised by the South Atlantic Anomaly, suggesting an intermittent or an absent behavior at the multi-millennial timescale.

Speleothems can provide high-quality continuous records of the direction and relative paleointensity of the geomagnetic field, combining high precision dating (with U-Th method) and rapid lock-in of their detrital magnetic particles during calcite precipitation. Paleomagnetic results for a mid-to-late Holocene stalagmite from Dona Benedita Cave in central Brazil encompass ~1900 years (3410 BP to 5310 BP, constrained by 12 U-Th ages) of paleomagnetic record from 58 samples (resolution of ~33 years). This dataset reveals angular variations of less than 0.06° yr

Paleomagnetic data provide information on the evolution of the Earth's magnetic field, and are used to reconstruct plate motions. One fundamental assumption underlying these interpretations is that the magnetization of a rock reliably records the direction and intensity of the magnetizing field, i.e. that the magnetization is parallel to the field direction, and the intensity of magnetization is proportional to the field strength. Preferred alignment or anisotropic distribution of magnetic grains can affect both the direction and the intensity of magnetization. Therefore, correction techniques, employing the anisotropy of magnetic susceptibility (AMS), thermal remanence (ATRM), or anhysteretic remanence (AARM) are used to account for these effects. We find that AARM within the same rock can vary dramatically with coercivity/grain size, so that anisotropy corrections can also depend on how AARM was measured. A consequence of the dependence of AARM on coercivity is that although a specimen may have been magnetized in a single direction, different grain size fractions may record magnetizations in different orientations. These directional variations, as revealed during progressive alternating field (AF) demagnetization, could erroneously be interpreted as changes in field or reorientation of the rock unit, when in reality they are related to grain-size-dependent remanence anisotropy. Similarly, intensity variations caused by grain-size-dependent anisotropy may bias paleointensity estimates. These observations have important consequences for studies on the evolution of the Earth's magnetic field, magnetic overprinting, and paleogeographic reconstructions.

Paleoclimate studies on speleothems commonly use oxygen isotopes as a record of precipitation variability and carbon isotopes to document soil, vegetation, and atmospheric processes. Magnetic miner...

The whole story An accurate, precise record of the carbon-14 ( 14 C) content of the atmosphere is important for developing chronologies in climate change, archaeology, and many other disciplines. Cheng et al. provide a record that covers the full range of the 14 C dating method (∼54,000 years), using paired measurements of 14 C/ 12 C and thorium-230 ( 230 Th) ages from two stalagmites from Hulu Cave, China. The advantage of matching absolute 230 Th ages and 14 C/ 12 C allowed the authors to fashion a seamless record from a single source with low uncertainties, particularly in the older sections. Science , this issue p. 1293

Collagen matrices are one form of artificial tissue that has applications in biomimetic organs or tumors, and in fundamental biology. Anatomical organs and tissues are often composed of aligned collagen, and in this study cross-linking nickel magnetic nanowires (MNWs) to collagen allowed a one-step bi-directional alignment of the collagen matrices when processed in a uniform magnetic field. These matrices were analyzed by differential interference contrast (DIC) microscopy, scanning electron microscopy (SEM) and polarized transmittance. The bi-directional alignment was also confirmed by plated, stained arachnoid cells from the blood-brain-barrier (BBB). Arachnoid cells are morphologically sensitive to their extracellular matrix (ECM) environment, and in this study, they were observed to spider out in two distinct directions as predicted by microscopy and transmittance. In fact, MNW-collagen matrices plated with arachnoid-cells are promising for future studies of artificial BBBs. Other cells (here osteosarcoma) have been observed to internalize MNWs, which leads to the possibility of barcoding matrices and cells with distinct signatures, pending a magnetic readout technique. To this aim, mixtures of two different MNW populations were analyzed using first order reversal curves (FORC), and the relative concentrations of the two populations were correctly estimated with negligible error for ratios of 1: 23 and only 7% error for ratios of 1: 115. Together, these studies open a path for magnetic identification of artificial tissues where distinct magnetic labels on matrices and in cells combine for a unique fingerprint.

Archaeomagnetic dating is a firmly established dating technique applicable to a wide variety of heat-treated anthropological materials and is advantageous for sites that lack materials suitable for radiocarbon dating. To correct recent misinterpretations of the method, we provide examples of how archaeomagnetic dating curves are calibrated and show how, in some instances, the technique can provide superior results. We emphasize that no single dating technique is capable of resolving the challenging chronology controversies in the Levant, and instead argue that multiple dating methods must be integrated in order to achieve the highest possible temporal resolution.

Hypervelocity impacts occur on bodies throughout our solar system, and play an important role in altering the mineralogy, texture, and magnetic properties in target rocks at nanometer to planetary scales. Here we present the results of hypervelocity impact experiments conducted using a two-stage light-gas gun with 5 mm spherical copper projectiles accelerated toward basalt targets with ~6 km s−1 impact velocities. Four different types of magnetite- and titanomagnetite-bearing basalts were used as targets for seven independent experiments. These laboratory impacts resulted in the formation of agglutinate-like particles similar in texture to lunar agglutinates, which are an important fraction of lunar soil. Materials recovered from the impacts were examined using a suite of complementary techniques, including optical and scanning electron microscopy, micro-Raman spectroscopy, and high- and low-temperature magnetometry, to investigate the texture, chemistry, and magnetic properties of newly formed agglutinate-like particles and were compared to unshocked basaltic parent materials. The use of Cu-projectiles, rather than Fe- and Ni-projectiles, avoids magnetic contamination in the final shock products and enables a clearer view of the magnetic properties of impact-generated agglutinates. Agglutinate-like particles show shock features, such as melting and planar deformation features, and demonstrate shock-induced magnetic hardening (two- to seven-fold increases in the coercivity of remanence Bcr compared to the initial target materials) and decreases in low-field magnetic susceptibility and saturation magnetization. © The Meteoritical Society, 2017. Russian Science Foundation, RSF: 17-17-01279 Rochester Academy of Science, RAS Acknowledgments—We thank the anonymous reviewer for the review and constructive suggestions and Prof. Oliver Tschauner for useful suggestions and the editorial handling, which helped to improve the manuscript. The work is supported by Act 211 Government of the Russian Federation, agreement no. 02.A03.21.0006 and is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University. We gratefully acknowledge support by the U.S. National Science Foundation IRM Visiting Fellowship to N.S.B. D.D.B. was supported by Program no. 7P of Presidium RAS and a Russian Science Foundation Grant 17-17-01279. D.D.B. acknowledges the staff at the Center of Microscopy and Nanotechnology of the University of Oulu (Finland) for assistance with some aspects of electron microprobe and SEM works.

Two flow-through experiments were conducted to assess serpentinization of intact dunite cores. Permeability and fluid chemistry indicate significantly more reaction during the second experiment at 200 °C than the first experiment at 150 °C. Permeability decreased by a factor of 2.4 and 25 during the experiments at 150 and 200 °C, respectively. Furthermore, hydrogen and methane concentrations exceeded 600 μmol/kg and 300 μmol/kg during the 200 °C experiment, and were one and two orders of magnitude higher, respectively, than the 150 °C experiment. Fe K-edge X-ray absorption near edge structure analyses of alteration minerals demonstrated Fe oxidation that occurred during the 200 °C experiment. Vibrating sample magnetometer measurements on post-experimental cores indicated little to no magnetite production, suggesting that the hydrogen was largely generated by the oxidation of iron as olivine was converted to ferric iron (Fe(III)) serpentine and/or saponite. Scanning electron microscopy images suggested secondary mineralization on the post-experimental core from the 200 °C experiment, portraying the formation of a secondary phase with a honeycomb-like texture as well as calcite and wollastonite. Scanning electron microscopy images also illustrated dissolution along linear bands through the interiors of olivine crystals, possibly along pathways with abundant fluid inclusions. Energy dispersive X-ray spectroscopy identified Cl uptake in serpentine, while Fourier transform-infrared spectroscopy suggested the formation of serpentine, saponite, and talc. However, no change was observed when comparing pre- and post-experimental X-ray computed tomography scans of the cores. Furthermore, (ultra) small angle neutron scattering datasets were collected to assess changes in porosity, surface area, and fractal characteristics of the samples over the ≈ 1 nm- to 10 μm-scale range. The results from the 200 °C post-experimental core generally fell within the range of values for the two pristine samples and the 150 °C post-experimental core that underwent negligible reaction, indicating that any change from reaction was smaller than the natural variability of the dunite. Even though there was little physical evidence of alteration, the initial stage of serpentinization at 200 °C was sufficiently significant to have a dramatic effect on flow fields in the core. Furthermore, this experiment generated significant dissolved hydrogen concentrations while simulating open system dynamics. Even though open systems prevent elevated hydrogen concentrations due to continual loss of hydrogen, we speculate that this process is responsible for stabilizing ferric Fe-rich serpentine in nature while also oxidizing more ferrous iron (Fe(II)) and cumulatively generating more hydrogen than would be possible in a closed system.

Anisotropy of magnetic susceptibility (AMS) indicates the preferred orientation of a rock's constituent minerals. However, other factors can influence the AMS, e.g. domain wall pinning or domain alignment in ferromagnetic minerals. Therefore, it is controversial whether samples should be alternating field (AF) demagnetized prior to AMS characterization. This may remove the influence of natural remanent magnetization (NRM) or domain wall pinning on AMS; however, it may also result in field-induced anisotropy. This study investigates the influence of stepwise AF and low-temperature demagnetization on mean susceptibility, principal susceptibility directions, AMS degree and shape for sedimentary, metamorphic and igneous rocks. Alternating fields up to 200 mT were applied along the sample x, y and z axes, rotating the order for each step, to characterize the relationship between AMS principal directions and the last AF orientation. The changes in anisotropy, defined by the mean deviatoric susceptibility of the difference tensors, are between

MAVEN magnetic field data indicate that a martian dynamo field was active 4.5 and 3.7 Ga ago., The absence of crustal magnetic fields above the martian basins Hellas, Argyre, and Isidis is often interpreted as proof of an early, before 4.1 billion years (Ga) ago, or late, after 3.9 Ga ago, dynamo. We revisit these interpretations using new MAVEN magnetic field data. Weak fields are present over the 4.5-Ga old Borealis basin, with the transition to strong fields correlated with the basin edge. Magnetic fields, confined to a near-surface layer, are also detected above the 3.7-Ga old Lucus Planum. We conclude that a dynamo was present both before and after the formation of the basins Hellas, Utopia, Argyre, and Isidis. A long-lived, Earth-like dynamo is consistent with the absence of magnetization within large basins if the impacts excavated large portions of strongly magnetic crust and exposed deeper material with lower concentrations of magnetic minerals.

Mercury and Earth are the only inner solar system planets with active, internally generated dynamo magnetic fields. The MESSENGER mission recently detected magnetic fields on Mercury that are consistent with lithospheric magnetization. We investigate the physical and chemical environment of Mercury's lithosphere, past and present, to establish the conditions under which magnetization may have been acquired and modified. Three factors are particularly crucial to the determination of crustal composition and iron mineralogy: redox conditions in the planet's crust and mantle, the iron content of the lithosphere, and, for any remanent magnetization, the temperature profile of the lithosphere and its evolution over time. We explore potential mechanisms for remanence acquisition and alteration on Mercury, whose surface environment is both hot and highly reducing. The long-term thermal history of Mercury's crust plays an important role in the longevity of any remanent crustal magnetization, which may be subject to remagnetization through thermal, viscous, and shock mechanisms. This thermal and compositional framework is used both to constrain plausible candidate minerals that could carry magnetic remanence on Mercury and to evaluate their capacity to acquire and retain sufficient magnetization to be detectable from satellite orbit. We propose that iron metal and its alloys are likely to be the dominant contributors to induced and remanent magnetization in Mercury's lithosphere, with additional contributions from iron silicides, sulfides, and carbides.

Magnetic minerals in paleosols hold important clues to the environmental conditions in which the original soil formed. However, efforts to quantify parameters such as mean annual precipitation (MAP) using magnetic properties are still in their infancy. Here, we test the idea that diagenetic processes and surficial weathering affect the magnetic minerals preserved in paleosols, particularly in pre-Quaternary systems that have received far less attention compared to more recent soils and paleosols. We evaluate the magnetic properties of non-loessic paleosols across the Paleocene–Eocene Thermal Maximum (a short-term global warming episode that occurred at 55.5 Ma) in the Bighorn Basin, WY. We compare data from nine paleosol layers sampled from outcrop, each of which has been exposed to surficial weathering, to the equivalent paleosols sampled from drill core, all of which are preserved below a pervasive surficial weathering front and are presumed to be unweathered. Comparisons reveal an increase in magnetization in outcrops compared with core equivalents, which is principally driven by secondary hematite production. Authigenic hematite production in outcrops presents a complication for goethite–hematite based paleoprecipitation proxies where estimates will be biased toward drier climate regimes. The occurrence of low coercivity minerals is more consistent between core and outcrop. However, we propose an alteration process for pedogenic magnetite that is observed in both core and outcrop, where pedogenic magnetite becomes progressively oxidized leading to higher mean coercivities and broader coercivity distributions compared to modern pedogenic magnetite. This combination of diagenetic processes and surface weathering influences the magnetic properties of paleosols. Despite these changes, magnetic enhancement ratios from B-horizons correlate with independent MAP estimates from geochemical proxies, which suggests that paleoprecipitation information is preserved. Future work should continue to address these complications by developing useful protocols that isolate the magnetic properties that are most resistant to alteration and remain strong indicators of MAP and climate.

Classification of lithic artifacts’ raw materials based on macroscopic attributes (e.g., color, luster, texture) has been used to pull apart knapping episodes in palimpsest assemblages by attempting to identify artifacts produced through the reduction of an individual nodule. These classes are termed “raw material units” (RMUs) in the Old World and “minimum analytical nodules” in the New World. RMUs are most readily defined for lithic artifacts in areas with distinctive cherts and other siliceous raw materials, allowing pieces from different nodules to be recognized visually. Opportunities to apply RMUs, however, are strongly limited at sites where lithic material visual diversity is low. The magnetic properties of obsidian, which result from the presence of microscopic iron oxide mineral grains, vary spatially throughout a flow. Consequently, obsidian from different portions of a source (i.e., different outcrops or quarries) can vary in magnetic properties. This raises the possibility that magnetic-based RMUs (mRMUs) for obsidian artifacts could be effective to distinguish individual scatters from multiple production episodes and offer insights into spatial patterning within a site or specific occupation periods. First, we assess the potential of mRMUs using obsidian pebbles from Gutansar volcano in Armenia. Second, we evaluate the validity of this approach based on a double-blind test involving an experimental assemblage of Gutansar obsidian flakes. Cluster analysis can successfully discern flakes from obsidian specimens containing high concentrations of iron oxides. Obsidian with more magnetic material has more opportunities for that material to vary in unique ways (e.g., grain size, morphology, physical arrangement). Finally, we apply the mRMU approach to obsidian artifacts from the Middle Palaeolithic site of Lusakert Cave 1 in Armenia and compare the results to traditional RMU studies at contemporaneous sites in Europe. In particular, we seek – but do not find – differences between retouch flakes (which reflect rejuvenation of tools) and the other small debris (which reflect other reduction activities). This result likely reflects the local landscape, specifically the abundance of obsidian and, thus, little pressure to curate and retouch tools. As this approach is applied to additional sites, such findings will play a central role in regional assessments about the nature and timing of the Middle to Upper Palaeolithic “transition” and the relationship, or lack thereof, between technological behaviors and presumed population dynamics.

International audience; Here we present a comprehensive magnetic characterization of synthesized non-ideal single-domain (SD) monoclinic pyrrhotite (Fe 7 S 8). The samples were in the form of a powder and a powder dispersed in epoxy. ''Non-ideal " refers to a powder fraction of predominantly SD size with a minor contribution of small pseudo-single-domain grains; such non-ideal SD pyrrhotite was found to be a remanence carrier in several types of meteorites (carbonaceous chondrites, SNC.. .), which justifies the usage of synthetic compositions as analogous to natural samples. Data were collected from 5 to 633 K and include low-field magnetic susceptibility (χ0), thermomagnetic curves, major hysteresis loops, back-field remanence demagnetization curves, first-order reversal curves (FORCs), alternating field and pressure demagnetization of saturation isothermal remanent magnetization (SIRM), low temperature data (such as zero-field-cooled and field-cooled remanence datasets together with room temperature SIRM cooling–warming cycles) as well as XRD and Mössbauer spectra. The characteristic Besnus transition is observed at $33 K. FORC diagrams indicate interacting SD grains. The application of hydrostatic pressure up to 2 GPa using nonmagnetic high-pressure cells resulted in the demagnetization of the sample by 32–38%. Repeated cycling from 1.8 GPa to atmospheric pressure and back resulted in a total remanence decrease of 44% (after 3 cycles). Pressure demagnetization experiments have important implications for meteorite paleomagnetism and suggest that some published paleointensities of meteorites with non-ideal SD monoclinic pyrrhotite as remanence carrier may be lower limits because shock demagnetization was not accounted for.

Meaningful interpretation of past human culture and political geography requires an accurate chronology that can be correlated with our modern calendar. The timing of seminal events during the Leva...

By observing the fluctuations in direction and intensity of the Earth's magnetic field through time, we increase our understanding of the fluid motions of the Earth's outer core that sustain the geomagnetic field, the geodynamo. Recent archaeomagnetic studies in the Near East have found extremely rapid increases – ‘spikes’ – in geomagnetic field intensity at ca. 3000 yr cal BP. These observations have proved problematic for our current understanding of core-flow. However, until now, these geomagnetic spikes had not been observed outside of the Near East, where they have been preserved in metallurgical slag and fired, mud brick walls. We present a new, fully oriented, geomagnetic secular variation and relative palaeointensity (RPI) record for the last 17,000 yr from Hall's Cave, Texas, whose complete, >3.8 m thick sedimentary sequence spans from the present to 16 , 850 ± 110 RC yr BP (Modern to 20,600 cal BP). Within the stable, cool climate of the cave, pedogenic and bioturbation processes are negligible to non-existent, thereby limiting post-depositional physical and geochemical alteration of the magnetic record. The sub-aerial and subterranean setting of the sedimentary sequence in Hall's Cave enabled us to collect oriented palaeomagnetic cubes from a previously excavated stratigraphic section. The palaeomagnetic samples yielded high-quality vectors. An age model for the sequence, determined using 15 AMS 14C-dates on individual bones from microvertebrates, was combined with the palaeomagnetic data to construct a secular variation record. The record is in broad agreement with predictions by Holocene field models for the site's location. However, starting ca. 3000 yr ago, the RPI data indicate an almost four-fold increase in geomagnetic field intensity lasting several hundred years. This record presents well-dated evidence, obtained using conventional techniques, for the existence of a geomagnetic intensity spike in North America that is contemporaneous with the shorter duration, decadal-scale spikes reported from the Near East.

Magnetic iron oxide minerals, principally magnetite, maghemite, hematite, and goethite are formed in well-drained soils in response to a suite of physical, chemical, and biological factors. Despite a wide range of complexity in the pedogenic processes that lead to magnetic mineral formation, dissolution, and transformation, there are well-documented empirical relationships between various magnetic mineral assemblages in soils with environmental and climatic conditions. Recently there has been an increase in the number of quantitative magnetic paleoprecipitation proxies that have been developed, and there is great potential for magnetic methods to be used in the geologic record to develop reconstructions of past climates. Magnetic paleoprecipitation proxies have been widely utilized in Quaternary or younger loess–paleosol systems; however, they have yet to be utilized in the pre-Quaternary fossil record. Future studies of magnetic mineralogy of soils and paleosols should aim to explore non-loessic modern soils and pre-Quaternary paleosols with more focus on understanding the interaction between magnetic mineral assemblages and soil moisture. Applications of existing and novel magnetic paleoprecipitation proxies in the fossil record should prove to be a valuable resource for paleoclimatologists.

Intermediate composition titanohematite, Fe2-yTiyO3 with 0.5 ≤ y ≤ 0.7, is seldom the focus of paleomagnetic study and is commonly believed to be rare in nature. While largely overlooked in magnetostratigraphic studies, intermediate titanohematite has been identified as the dominant ferrimagnetic mineral in an array of Late Mesozoic and early Cenozoic Laramide clastic deposits throughout the central United States. Intermediate titanohematite is ferrimagnetic and has similar magnetic properties to titanomagnetite, except its unique self-reversing property. Due to these similarities, and with detrital remanent magnetizations masking its self-reversing nature, intermediate titanohematite is often misidentified in sedimentary deposits. Past studies relied upon nonmagnetic techniques including X-ray diffraction and electron microprobe analysis. While these techniques can identify the presence of intermediate titanohematite, they fail to test whether the mineral is the primary recorder. To facilitate the identification of intermediate titanohematite in sedimentary deposits, we characterize this mineral using low-temperature magnetometry and high-temperature susceptibility experiments, and present a new identification technique based on titanohematite's self-reversing property, for sediments that span the Cretaceous-Paleogene boundary (Hell Creek region, Montana). Results from the self-reversal test indicate that the majority of remanence is held by minerals that become magnetized parallel to an applied field, but that intermediate, self-reversing titanohematite (y = 0.53–0.63) is an important ancillary carrier of remanence. While earlier literature suggests that intermediate titanohematite is rare in nature, reanalysis using specialized rock magnetic techniques may reveal that it is more abundant in the rock record, particularly within depositional basins adjacent to calc-alkaline volcanics, than previously thought.

The age and paleoenvironmental record of lacustrine deposits in the Aktoprak basin of south-central Turkey provides information about the evolution of topography, including the timing of development of an orographic rain shadow caused by uplift of the mountain ranges fringing the Central Anatolian Plateau. New magnetostratigraphy-based age estimates, in combination with existing biostratigraphic ages, suggest that the partially remagnetized Kurtulmus Tepe section of the basin is Chattian (Upper Oligocene). The mean carbon and oxygen stable isotope ratios (δ18O= 24.6 ± 2.0 ‰, δ13C= −4.9 ± 1.1‰) are largely constant through the section and indicative of a subtropical, open freshwater lake. These isotopic values are also similar to those of the Chattian Mut basin to the south, on the Mediterranean side of the modern orographic barrier (Tauride Mountains), and indicate absence of an orographic barrier during Late Oligocene basin deposition. Post-depositional partial remagnetization occurred after tilting of the basin sequence and was mineralogically controlled, affecting grey, carbonate-rich rocks (average %CaCO3= 82), whereas interlayered pink carbonate-poor rocks (average %CaCO3= 38) carry a primary, pretilt magnetization. The pink rocks are rich in clay minerals that may have reduced the permeability of these rocks that carry a primary magnetization, concentrating basinal fluid flow in the carbonate-rich grey layers and leading to the removal and reprecipitation of magnetic minerals. The normal and reverse polarities recorded by the remagnetized rocks suggest that remagnetization occurred over a protracted period of time.

In regions that lack built structures or stratified open archaeological sites, such as precolonial Australia, rockshelters are a major source of detailed information for understanding the nature and timing of human occupation. A key concern is that the proposed ages for the earliest archaeological sites are based on luminescence dating of sediments, rather than directly of cultural materials, leaving the association between the sediments and evidence of human activity questionable. Here, we present evidence of magnetic enhancement associated with cultural horizons within the deposits of a Pleistocene rockshelter in interior northern Queensland. Soil magnetic studies combined with experimental burning show that magnetically enhanced sediments in Gledswood Shelter 1 are the result of anthropogenic burning of hearth fires, which burn hotter and for a longer time than natural wild fires. These techniques appear to work in this setting because of the nature of the local geology and the geological antiquity of the landscape. Susceptibility and frequency dependence of susceptibility signatures provide a critical tool to resolve that human occupation starts at 2.2 m depth within a stratigraphic section. In conjunction with luminescence dating, soil magnetic studies provide an opportunity for archaeologists to resolve the timing of human settlement in Australia and other intracratonic plate settings.

Strategies employed by Middle Palaeolithic hominins to acquire lithic raw materials often play key roles in assessing their movements through the landscape, relationships with neighboring groups, and cognitive abilities. It has been argued that a dependence on local resources is a widespread characteristic of the Middle Palaeolithic, but how such behaviors were manifested on the landscape remains unclear. Does an abundance of local toolstone reflect frequent encounters with different outcrops while foraging, or was a particular outcrop favored and preferentially quarried? This study examines such behaviors at a finer geospatial scale than is usually possible, allowing us to investigate hominin movements through the landscape surrounding Lusakert Cave 1 in Armenia. Using our newly developed approach to obsidian magnetic characterization, we test a series of hypotheses regarding the locations where hominins procured toolstone from a volcanic complex adjacent to the site. Our goal is to establish whether the cave's occupants procured local obsidian from preferred outcrops or quarries, secondary deposits of obsidian nodules along a river, or a variety of exposures as encountered while moving through the river valley or across the wider volcanic landscape during the course of foraging activities. As we demonstrate here, it is not the case that one particular outcrop or deposit attracted the cave occupants during the studied time intervals. Nor did they acquire obsidian at random across the landscape. Instead, our analyses support the hypothesis that these hominins collected obsidian from outcrops and exposures throughout the adjacent river valley, reflecting the spatial scale of their day-to-day foraging activities. The coincidence of such behaviors within the resource-rich river valley suggests efficient exploitation of a diverse biome during a time interval immediately preceding the Middle to Upper Palaeolithic "transition," the nature and timing of which has yet to be determined for the region.

Large amplitude aeromagnetic and gravity anomalies over a ∼16,000 km2 area of northeast Iowa and southeast Minnesota have been interpreted to reflect the northeast Iowa intrusive complex (NEIIC), a buried intrusive igneous complex composed of mafic/ultramafic rocks in the Yavapai Province (1.8-1.7 Ga). Hundreds of meters of Paleozoic sedimentary cover and a paucity of basement drilling have prevented detailed studies of the NEIIC. Long considered, but not proven, to be related to the ∼1.1 Ga Midcontinent Rift System (MRS), the NEIIC is comparable in areal extent to the richly mineralized Duluth Complex and is similarly located near the margin of the MRS. New geochronological and geophysical data together support an MRS affinity for the NEIIC. A dike swarm imaged in aeromagnetic data is cut by intrusions of the NEIIC, and a new apatite U-Pb date of ∼1170 Ma on one of the dikes thus represents a maximum age for the NEIIC. A minimum age constraint is suggested by (1) large-volume magmatism associated with the MRS that was the last such event to affect the region; and (2) the presence of reversely magnetized dikes, similar in character to MRS-related dikes elsewhere, that cut several intrusions of the NEIIC. The NEIIC is largely characterized by the presence of multiple zoned intrusions, many of which contain large volumes of mafic-ultramafic rocks and have strong geophysical similarities to alkaline intrusive complexes elsewhere, including the MRS-related Coldwell Complex of Ontario. The largest of the zoned intrusions are ∼40 km in diameter and are interpreted to have thicknesses of many kilometers. Suspected faults, alignments of intrusions, and intrusive margins tend to be aligned along northwest and northeast trends that match the trends of the Belle Plaine fault zone and Fayette structural zone, both previously interpreted as pre-MRS, possibly lithospheric-scale discontinuities that may have controlled NEIIC emplacement. These interpretations collectively imply notable potential for the NEIIC to host several different types of undiscovered base metal and critical mineral deposits.

Flooding events are major natural hazards that present significant risk to communities worldwide. Calculations of flood recurrence rate through time are important tools for regulating land use, determining insurance rates, and for the design and construction of levees and dams. Typically, flood recurrence rates are based on limited historical data or on evidence preserved in the geologic record as overbank deposits, tree ring scars, or high water scour marks. However, these approaches are either limited in their ability to produce continuous time series of flooding events or do not consider the effects of regional land use change. Here we use scanning superconducting quantum interference device (SQUID) microscopy to rapidly image the magnetization associated with flood layers in a polished surface of an annually laminated stalagmite from Spring Valley Caverns (SVC) in southeastern Minnesota. A time series of magnetization peaks, each of which corresponds to a flooding event, yields an average flood recurrence rate of ≤5 events per century for the last 500 years. This rate increases to ∼7 events per century since 1900, coincident with historical timber and agricultural land-use changes in Minnesota. This approach produces a continuous record of well-dated, extreme-precipitation events that can be examined within the context of land use change.

Paired measurements of

Speleothems are important paleoclimate archives. Researchers typically compile measurements of stable isotopic ratios dated using high precision U-Th radiometric techniques to reconstruct regional and global climate. Magnetic material incorporated within speleothems can provide an independent means of connecting large-scale climatic changes with their impact on more localized processes in soils overlying cave systems. Under certain environmental conditions, pedogenic processes can produce magnetite nanoparticles. Enhancement of pedogenic magnetite in soil profiles depends strongly on local precipitation. Pedogenic magnetite can be subsequently transferred via drip-waters into underlying cave-systems and incorporated into speleothems as they grow. Here, we employ high-resolution magnetic methods to analyze a well-dated stalagmite from Buckeye Creek Cave, West Virginia (USA), and find that changes in magnetite concentration follow both changes in stable isotopes measured in the same stalagmite and global climate proxies. We interpret the changes in magnetite concentration as reflecting variations in local pedogenic processes, controlled by changes in regional precipitation. This record demonstrates how magnetic measurements on speleothems can constrain interpretations of speleothem climate proxies.

The Glass Buttes complex in the High Lava Plains of Oregon produced obsidians during a series of rhyolitic eruptions circa 5.8 to 6.5 Ma. These obsidians have been used to craft stone tools for millennia, from Clovis peoples to modern knappers, and have been recovered at sites throughout the Pacific Northwest. Glass Buttes is also the origin of much obsidian used for lithic replication experiments and to test new techniques for sourcing. Paradoxically, the spatial distributions of chemically distinct obsidians at this complex have received comparatively little attention. The only published study to connect obsidian compositional differences to the Glass Buttes landscape is Ambroz et al. (2001), who reported seven discrete, spatially constrained obsidian “chemical groups” based on clustered data in elemental scatterplots. Since its publication, their paper has been cited as an example of successful obsidian “subsource” characterization. During the course of a wider research program, we collected 337 specimens at Glass Buttes from loci originally sampled by Ambroz et al. (2001). While we could replicate the “chemical groups” observed by Ambroz et al. (2001), we were unable to reproduce the reported spatial distribution of sources across the landscape. Almost half of the resampled loci exhibited intermingled populations of chemical types due to their locations on alluvial–colluvial deposits. We also identified five additional obsidian chemical types at the complex; however, geochemically significant elements suggest that only a subset of these eleven types correspond to different flows and domes at Glass Buttes. A few appear to reflect chemical zoning within flows, whereas two other types may be exogenous, moved from nearby obsidian sources via secondary transport. Thus, we demonstrate here that the discrete subsource zones demarcated by Ambroz et al. (2001) do not reflect reality on the landscape. This, in turn, alters how artifact sourcing results are interpreted with respect to how mobile foraging groups interacted with the landscape and made choices regarding resource acquisition, toolstone provisioning, and land use.

Old minerals expose an ancient field Mercury is the only terrestrial planet other than Earth with an active, internally generated magnetic field. Results from the MESSENGER spacecraft indicate that the field is almost as old as the planet. Johnson et al. took advantage of close flybys to extract evidence of an ancient magnetic field. Certain minerals are able to “lock in” the signature of a field at the time they crystallize. This remnant magnetization was found in a region on Mercury believed to be 3.8 billion years old. Science , this issue p. 892

Large-amplitude gravity and magnetic highs over northeast Iowa are interpreted to reflect a buried intrusive complex composed of mafic–ultramafic rocks, the northeast Iowa intrusive complex (NEIIC)...